The gasification of coal by reaction with steam and air has been practiced for more than a century. For most of those years it was produced in relatively simple equipment at a pressure slightly higher than atmospheric pressure. Gas produced through coal gasification has been used to provide fuel gas for lighting and heating. In such simple gasification processes, by-products of coke, tars and light oils were produced in sufficient quantities that their values influenced the selling price of the gas. In recent years many of the outlets for the by-products have been replaced by electricity, natural gas and petroleum distillates. In order to compete with electricity, natural gas, and petroleum distillates, more sophisticated equipment for the gasification of coal has been designed and developed. Such newer sophisticated coal gasification processes operate at high pressures and use, as reactants, steam and pure or nearly pure oxygen. Such equipment is costly and complex to operate with the result that such operating units must be large to achieve good economics.
At the present time power plants for the production of electricity are large and the electricity is distributed through nation-wide networks. There is, however, still a need for plants of all sizes to supply local demands for power and cogeneration. Irrespective of the size of the plant, plant emissions must meet environmental requirements set by governmental authorities. These factors have led to difficulties in the use of coal as the primary fuel for power generation or co-generation plants.
The design of power plants has also changed with the development of the gas turbine as a highly efficient power unit, especially when combined with a heat recovery steam generator and steam turbine. Gas turbines, however, operate solely on gaseous or liquid fuels. Clean fuel gas produced by large coal gasifiers, in conjunction with clean up equipment, may be used in generating power with gas turbines. Large gasifiers which can convert from 1,200 to 2,500 tons of coal per day to gas and provide fuels to gas turbine combined cycle plants of 250 to 500 megawatt capacity are known.
Although there is a demand for gas turbine based power plants in that size range, gas produced from coal has not generally been used to produce electrical power because the complexity of the equipment and operating costs of oxygen based systems make the use of coal economically impractical. However, for many decades small and medium sized gasifiers have been used to provide fuel gas, at low pressure, to such plants as brick works, glass plants and lime kilns. Such small and medium size gasifiers use air as a reactant and so the quality of the gas produced is lower than that of the large plants that use oxygen, particularly as regards gas heating value and composition. Because the operating pressure is low the normal method for producing the air necessary for the gasification is through the installation of air blowers. The use of air blowers is very inefficient with the result that the gasification process has a low efficiency. In cases where the quantity of air is large and the pressure higher, axial flow compressors, which have high efficiencies, can be used. Such is the case with the compressors used in gas turbine engines.
The gas turbine consists of a highly efficient air compressor that supplies air to a combustion chamber in which fuel is burned. The hot gas produced flows into an expander where it produces work. This expander is connected to the compressor and also to a mechanical drive commonly to an electricity generator. It is usual to supply the fuel in the form of natural gas or fuel oil, which have heating values on the order of 21,000 British Thermal Units per pound and the fuel supplied amounts to slightly less than 2% of the air supplied. The heating value of coal gas is about 3,000 British Thermal Units per pound and to achieve the same gas temperature in the gas turbine the amount of fuel added must be at least 14% of the airflow. This leads to a substantial increase in gas flow through the expander and a consequent increase in the power produced. This higher power, in some cases, is higher than the value that can be transmitted to the electricity generator without exceeding the mechanical limits of the drive shaft. The normal method of preventing this condition is to reduce the airflow to the compressor and hence the flow through the turbine.